1. Field of the Invention
This invention relates generally to improvements in digital audio processing and specifically to a system and method for implementing a refined psycho-acoustic modeler in digital audio encoding.
2. Description of the Background Art
Digital audio is now in widespread use in audio and audiovisual systems. Digital audio is used in compact disk (CD) players, digital video disk (DVD) players, digital video broadcast (DVB), and many other current and planned systems. A problem in all of these systems is the limitation of either storage capacity or bandwidth, which may be viewed as two aspects of a common problem. In order to fit more digital audio in a storage device of limited storage capacity, or to transmit digital audio over a channel of limited bandwidth, some form of digital audio compression is required.
Because of the structure of digital audio, many of the traditional data compression schemes have been shown to yield poor results. One data compression method that does work well with digital audio is perceptive encoding. Perceptive encoding uses experimentally determined information about human hearing from what is called psycho-acoustic theory. The human ear does not perceive sound frequencies evenly. It has been determined that there are 25 non-linearly spaced frequency bands, called critical bands, to which the ear responds. Furthermore, it has been shown experimentally that the human ear cannot perceive tones whose amplitude is below a frequency-dependent threshold, or tones that are near in frequency to another, stronger tone. Perceptive encoding exploits these effects by first converting digital audio from the time-sampled domain to the frequency-sampled domain, and then by not allocating data to those sounds which would not be perceived by the human ear. In this manner, digital audio may be compressed without the listener being aware of the compression. The system component that determines which sounds in the incoming digital audio stream may be safely ignored is called a psycho-acoustic modeler.
A common example of perceptive encoding of digital audio is that given by the Motion Picture Experts Group (MPEG) in their audio and video specifications. A standard decoder design for digital audio is given in the MPEG specifications, which allows all MPEG encoded digital audio to be reproduced by differing vendors' equipment. Certain parts of the encoder design must also be standard in order that the encoded digital audio may be reproduced with the standard decoder design. However, the psycho-acoustic modeler may be changed without affecting the ability of the resulting encoded digital audio to be reproduced with the standard decoder design.
Early consumer products using MPEG standards, such as DVD players, were playback-only devices. The encoding was left to professional studio mastering facilities, where shortcomings in the psycho-acoustic modeler could be overcome by making numerous attempts at encoding and adjusting the equipment until the resulting encoded digital audio was satisfactory. Moreover the cost of the encoding equipment to a recording studio was not a substantial issue. These factors will no longer be true when newer consumer products, such as recordable DVD players and DVD camcorders, become available. The consumer will want to make a satisfactory recording with a single attempt, and the cost of the encoding equipment will be a substantial issue. Therefore, there exists a need for a refined psycho-acoustic modeler for use in consumer digital audio products.